1. Technical Field
This invention relates in general to communication devices and, more particularly, to a fully digital method and apparatus for amplitude and phase modulation.
2. Description of the Related Art
Quadrature modulation is commonly used for communication. FIG. 1 shows a conventional prior art direct up-conversion transmitter 10. The in-phase (I) and quadrature (Q) pulse-shaped digital baseband signals from the digital baseband 12 are converted into analog domain with digital-to-analog (D/A) converters 14. Due to their digital nature, the D/A outputs exhibit strong sampling-time harmonics and switching noise, which have to be conditioned with low-pass filters (LPF) 16 before being up-converted to the RF carrier by a modulator 18, which is a critical RF/analog block. The RF frequency synthesizer 20 is used as a local oscillator (LO) in the transmitter 10 to perform frequency translation. The power amplifier (PA) 22 is the last stage of the transmitter path. The power amplifier performs antenna impedance matching and brings the emitted signal to the required power level for transmission to the antenna 24.
A major weakness of this mixer-based transmitter architecture is that even a small mismatch in phase shift or amplitude gain between the I and Q paths can significantly impair the system performance. Furthermore, because of a certain amount of inherent frequency shift between the modulator input and output (it performs frequency translation by design), the strong power amplifier signal can cause frequency pulling of the oscillator 20 through injection locking. This mechanism finds parasitic paths, such as substrate, power and ground lines as well as electromagnetic radiation to feed strong power amplifier signal into most sensitive parts of the oscillator.
Accordingly, the analog sections of the transmitter 10 require significant component matching for accurate performance and is not amenable to a deep-submicron CMOS implementation.
Therefore, a need has arisen for an quadrature modulation circuit which can be efficiently implemented with CMOS fabrication techniques.